1. Field of the Invention
The present invention relates to an optical module.
2. Description of the Related Art
An optical reception module or optical transmission module used in the present market of optical communications has generally the form of a coaxial type package or box type package. In the package, a light-receiving element or/and a light-emitting element, and a modulating element and an amplifier IC or the like, if needed, are contained. Moreover, a lens, a fiber, and the like are accommodated in the package for inputting/outputting an optical signal from/to the outside. For example, in the optical reception module, the beam spot of an optical signal input through an optical fiber is narrowed by a lens, and thereafter converted to an electric signal through photo electric conversion at an absorbing layer (light-receiving portion) of a light-receiving element.
This electric signal is generally as feeble as approximately several microamperes to several milliamperes. Therefore, the electric signal is amplified in a transimpedance amplifier IC, and output as a voltage signal from the optical reception module.
In recent years, a communication capacity required for per optical module is increased with an increase in communication rate, so that optical transmission/reception modules having a transmission rate of approximately 40 Gbps or 100 Gbps are demanded. In the optical transmission/reception modules, a multichannel system such as 10 Gbps×4 ch or 25 Gbps×4 ch may be used, and, for example, a multichannel optical module that inputs/outputs modulated optical signals at four wavelengths is known.
In the optical module, an optical element including reception portions or light-emitting portions corresponding to the number of a plurality of channels, or a driver IC, transimpedance IC, or the like including amplifiers corresponding to the number of the plurality of channels is mounted.
Therefore, input/output electric signals corresponding to the plurality of channels are needed, and also the numbers of power supplies, grounds, biases, and inputs/outputs of a control signal are great. In the optical module, electrical connection between the interior of the package and the outside is generally made through a transmission line pattern wired on a ceramic substrate attached to the package and a flexible board connected to the transmission line pattern with solder or the like. An example of such an optical module is disclosed in JP 2012-018289 A.
An optical module disclosed in JP 2012-018289 A is configured such that one flexible board is bent in a U shape near the substantial center thereof, and that a first pattern and a second pattern are patterned on the facing flexible board.
The optical module using the flexible board is one of configurations best suited for the multichannel optical module. That is, it is possible to dispose input/output electric signals corresponding to the plurality of channels on the flexible board and configure patterns for the plurality of power supplies, grounds, biases, or control signals on the flexible board.
Here, it is generally desired in a high-speed optical module to suppress, for example, crosstalk that causes malfunction. For example, JP 2012-018289 A, in which the first pattern and the second pattern are wired on the same flexible board, discloses a technique for suppressing crosstalk between these first wiring pattern and second wiring pattern.
Specifically, it is disclosed that a portion of a grounding conductor pattern is located between the first wiring pattern and the second wiring pattern facing the first wiring pattern when the flexible board is bent in a U shape. Thus, the crosstalk between the first wiring pattern and the second wiring pattern is suppressed.
In the optical module described above, the technique for suppressing the crosstalk between the first wiring pattern and the second wiring pattern is insufficient, and thus involves the problem of malfunction or the like of the optical module due to noise or the like.
The reason is as follows. Both the first wiring pattern and the second wiring pattern need to be connected using a conductive adhesive to a feedthrough portion constituting the optical module. Therefore, the first wiring pattern and the second wiring pattern are close to each other in the vicinity of their portions fixed at the feedthrough portion, and thus electrical crosstalk is caused in the vicinity of the portions.
For maintaining the fixing strength between the wiring pattern and the feedthrough, the conductor width of the wiring pattern generally needs to be widened to increase the bonding area. Actually, in the known example described above, a connection pad of the feedthrough is greater than the wiring pattern of the flexible board.
That is, the pad size of the first wiring pattern and the second wiring pattern is increased in the vicinity of the position at which the first wiring pattern and the second wiring pattern are fixed to the feedthrough, the wiring patterns are close to each other, and thus electrical crosstalk is caused in the vicinity of the position.